There isn't a general stats model yet. What I've been doing is using the APIC-EM inventory to get CLI/SNMP credentials and using direct device access to grab additional information. As we see RESTCONF/NETCONF take shape, that will be an alternative to gather operational data until models are added to APIC-EM itself.
Sorry I wasn't clear. I did not use APIC-EM to send CLI commands directly. I used APIC-EM to provide my script credentials, and then I was able to login (using the Python paramiko module) to get the additional data. If you need to use an API, and you have Prime Infrastructure, then Adam's suggestion is a good one.
The other way you could do this today, is to use the Prime Infrastructure API. For example to get tx statistics, you could use the following request. By default it returns the samples for the past hour, but you can get historical as well.